Velocity measurement of a moving masses is necessary for many industrial operations and in many applications noncontact velocity measurement is preferred or required. For example, a noncontact measurement is required where a contacting measurement device would interfere with the normal function or operation of the moving mass, where space on or around the mass is limited, or where the moving mass cannot accept any foreign objects. The present invention is an improved noncontact apparatus of method for measuring such velocity, and as used herein, the term "noncontact" is used to mean that there is no mechanical contact with the moving mass and no object or other material is attached to or introduced into the moving mass.
Many contact and noncontact methods of measuring the velocity of a moving mass are known. One of the most common types of velocity measurement involves the measurement of some property of a moving mass at two points along the direction of movement. For example, the reflective properties, the temperature or the density of the moving mass may be examined at two points to determine velocity. These methods usually depend on the existence of fluctuations in those properties (off average conditions) around their average values and the propagations of small volumes of these off average conditions for some distance along the direction of movement or flow. These off average conditions may be analyzed at two points to determine transit time. For example, in one such device two light sources and two light sensors are used to monitor a moving mass at two different points along the travel path of the moving mass. One source and sensor monitor reflective light from the mass at one point along the travel path and the other source and sensor monitor reflective light at another point which is down stream from the first point. Because of surface inhomogeneities, the reflected light will be different when reflected from different positions on the moving mass. However, since the two source and sensor pairs are monitoring the same moving mass, and assuming that surface conditions do not signficantly change between the first and second positions, the signals produced by each pair should be the same or similar, but time shifted one with respect to the other. The time shaft between these two signals is analyzed to determine the transit time between the two points and, thus, determine velocity.
In another method, a detectable object, such as a reflector, is placed on a moving object, usually a rotating object, and it is illuminated. The light reflected from the moving reflector is then analyzed to determine velocity.
In yet another velocity measurement technique that is commonly used to detect fluid flow velocity, a heater is placed into the flowing fluid and the heater is modulated so that the heat produced is changing over a period of time. A heat sensor is disposed in the fluid downstream from the heater to detect the heat fluctuations in the fluid and to produce a signal. The signal produced by the heat sensor will be time shifted with respect to the modulation of the heater, and this time shift may be analyzed to determine the transit time of the fluid between the heater and the sensor.
While known techniques for measuring velocity of moving masses have been adequate for many applications, the present invention generally offers significant advantages over such techniques and is more flexible in terms of the applications for which it is suited. For example, placing a temperature probe and a heater in a moving stream of fluid may cause turbulence or other undesirable disturbances in the fluid flow. These disturbances may create inaccuracies in flow measurement or have other undesirable consequences unrelated to velocity measurement. The present invention avoids creating such disturbances by not contacting the moving mass. Also, in most cases, it would not be practical to insert heaters and heat sensors in a flow of solid objects or a slurry flow. The sensors would impede the movement or flow of the objects, and the solid objects may destroy the heaters and sensors. Likewise, these problems are overcome in the present invention because it does not require any type of contact with the moving mass.
The reflected light technique described above depends on the reflective characteristic of the moving mass which usually means that it depends on the surface characteristics thereof. In some situations the surface characteristics are changing very rapidly, and if these changes are sufficient, it may be impossible to correlate signals generated by reflections from the moving mass at two points along the travel path. Since the present invention does not rely on surface characteristics, this problem is avoided.
In accordance with the present invention, an apparatus for measuring the velocity of a moving mass having a direction of travel includes a noncontact heat source. The heat source intermittently heats and creates a hot spot on the moving mass at a first position in space, and during the heating operation, the heat source remains out of contact with the moving mass. A noncontact detector is also provided for detecting heat at a second position in space, and during the detection operation, the detector remains spaced apart from the second position. The second position is spaced a predetermined distance apart from the first position and, with respect to the first position, the second position is disposed in the direction of travel of the moving mass. As the mass passed by, the detector is operable to detect and generate a detection signal when the hot spot is in the second position. A monitor system is provided for monitoring when a hot spot is created on the moving mass in the first position and for monitoring the detection signal to determine when the hot spot is in the second position. The monitor system then generates a monitor signal that corresponds to the time elapsed while the hot spot on the moving mass travels from the first position to the second position, and, thus, corresponds to the velocity of the moving mass.
In the preferred form, the noncontact heat source is an infrared light source and the noncontact heat detector is an infrared heat detector. Also, in order to improve reliability, a band pass filter is used to condition the detection signal. Background light and heat sources will be detected by the detector and in response to this background noise, a low frequency signal will be generated. In contrast, the hot spot on the moving mass when detected by the detector, will cause a sudden jump in the detection signal and will have a high frequency. The band pass filter is chose to pass the relatively high frequency signals created when the hot spot enters the second position and to reject the relatively low frequency signal created by background noise.
In accordance with another aspect of the preferred form of the present invention, the velocity of the moving object is determined by looking at several transit times of several hot spots moving from the first to the second positions. This plurality of transit times is processed using a cross correlation function to calculate the velocity of the moving mass.
In accordance with yet another aspect of the preferred form of the present invention, a second infrared light detector may be used to determine when the heat source is creating a hot spot. This infrared light detector would simply monitor the output of the infrared heat source and would provide a signal as part of the monitor system to indicate when the hot spot is being created. Alternatively, the monitor system could simply monitor the power to the radiant heat source and use the power signal to determine when the heat source is creating the hot spot.
In the preferred form of the present invention, the detector includes a micrometer mounting system for adjusting the position of the detector so that the distance between the heat source and the detector may be precisely maintained. An electronic caliper is provided to constantly monitor the position of the heat detector and provides such position information back to the monitor system. Finally, as an aid to the positioning of the heat detector, an aiming light is mounted on the heat detector to transmit a beam of light to indicate the path and position that will be detected by the heat detector.
In addition to the apparatus described above, the invention encompasses the methods that are performed by the above described apparatus for measuring the velocity of a moving mass.